The invention relates to forming and filling flexible plastic packaging.
In form and fill packaging apparatus, a web of plastic is advanced through a vacuum thermoformer (where the plastic web is formed to provide receptacles for receiving articles), a filling station (where articles are placed in the formed receptacles), a sealer (where a second sheet is placed over the filled receptacles and sealed to portions of the web around the receptacles to provide covers), and a cutter (where the plastic web is cut at portions between receptacles into separate packages).
In such apparatus different thermoforming methods have been employed, e.g., negative forming (in which a vacuum in a female mold draws heated film into the desired shape against the female mold, with or without assistance from positive pressure on the other side of the plastic), positive male forming (in which a vacuum in a male mold and positive pressure applied on the other side of the plastic draw the heated film into the desired shape against the male mold), and male plug-assisted negative forming (in which a vacuum in a female mold and a mechanical male plug on the opposite side of film are used, with or without an optional sandwich heater).
Such apparatus has been used to form both rigid and flexible receptacles, the former in general having walls that are 8 to 50 mils in thickness (depending on the plastic and application) and maintain their shape, the latter in general having walls that are less than 8 mils in thickness and readily flex and change shape. The differences in size and properties of rigid vis-a-vis flexible walls result in differences in the responses of the plastics to heat and force and in the types of thermoforming procedures employed. Flexible film thermoforming conventionally involves simple application of vacuum to easily draw the heated plastic to the mold; there often is, however, substantial stretching of the plastic, resulting in uneven reduction in wall thickness. Rigid wall thermoforming may very well involve procedures employing assistance from positive pressure in addition to vacuum and has also involved a two-step procedure of first providing a light vacuum to provide initial contour and thereafter providing both vacuum and pressure to force the film into the shape of the mold.